1. Field of the Invention
The present invention relates to a cleaning device that removes a toner remaining on an image carrier of an image forming apparatus after image transfer.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus, while an image carrier is rotating, a charging device uniformly charges a peripheral surface of the image carrier, an exposing device performs writing to form an electrostatic latent image on the image carrier, a developing device deposits a toner to visualize the electrostatic latent image, and forms a toner image on the image carrier. A transfer device transfers the toner image to a recording medium, and a fixing device fixes the transferred image onto the recording medium. After the transfer of the toner image, a cleaning device cleans the peripheral surface of the image carrier to prepare for the next image formation.
A cleaning blade made of an elastic material like polyurethane rubber is used as a cleaning member in the cleaning device so as to simplify the structure and achieve excellent cleaning performance. In the cleaning device, a support member supports a base end of the cleaning blade, and presses a tip ridge portion of the cleaning against the peripheral surface of the image carrier to scrape off a toner remaining on the image carrier.
Meanwhile, there has been an increasing demand for improvement of image quality in this type of electrophotographic image forming apparatus. To meet such demand, toner particles have been reduced in size and made spherical. Accordingly, a spherical toner formed by using a polymerizing method has become mainstream.
However, when toner particles are reduced in size and made spherical, it is difficult to completely remove a residual toner with the cleaning blade. This is because a rotation moment is generated in the toner at a position on the image carrier against which the cleaning blade is pressed. The rotation moment pushes-the cleaning blade up to let the toner to slip into a space between the cleaning blade and the image carrier.
Therefore, when the toner having small and spherical toner particles is used, it is necessary to increase pressing force of the cleaning blade against the image carrier to prevent the toner from slipping into the space between the cleaning blade and the image carrier. In general, “linear pressure” has been used to represent a force for preventing the toner from slipping into the space under the cleaning blade. The “linear pressure” [gf/cm] calculated by dividing a total load applied to the cleaning blade by a length of the tip ridge portion of the cleaning blade pressed against the image carrier.
Specifically, a tip of the cleaning blade is pressed against the image carrier such that the tip portion of the blade comes into a stick state. A sheet-like sensor with a thickness of 0.1 millimeter is placed in a position where the cleaning blade is pressed against the image carrier. The “linear pressure” is calculated by dividing output of the sensor (a load [g] acting on the position) by a length [cm] in an axial direction of the image carrier of the position.
Note that the sheet-like sensor includes a large number of electrodes arranged in two directions (a row direction and a column direction) orthogonal to each other. Surfaces of the electrodes are covered with film resin. In the electrodes, a pressure sensitive resistive substance and a charge generating substance are set in a lattice shape. When an external pressure is applied to intersections of the lattice shape, resistance changes according to a load of the external pressure. The change in the resistance appears as a change in a current flowing in the row direction and the column direction. Thus, a total load is calculated from the current.
An increased “linear pressure” improves cleaning performance for a toner having small and spherical toner particles that are difficult to clean off. However, an increase in linear pressure causes harmful effects. For example, abrasion of the image carrier increases, torque of the image carrier increases, and abrasion of the cleaning blade increases.
Moreover, the ability of preventing a toner from slipping into the space under the cleaning blade cannot be sufficiently evaluated with the linear pressure. In reality, a nip is formed between the cleaning blade and the image carrier at the position where the cleaning blade is pressed against the image carrier. Specifically, the cleaning blade is not in line contact, but in surface contact with the image carrier. However, as described above, the “linear pressure” is calculated by dividing a total load applied to the cleaning blade by a length in an axial direction of the image carrier of the position where the cleaning blade is pressed against the image carrier. Therefore, a contact area of the cleaning blade with the image carrier is not taken into account at all.
Thus, it is not always possible to clean the toner having small and spherical toner particles simply by increasing the “linear pressure”. Instead, harmful effects are caused by an increased linear pressure.
One approach is to use a “surface pressure” as a characteristic representing a force for preventing a toner from slipping into the space between the cleaning blade and the image carrier. The surface pressure is calculated by dividing a total load applied to the cleaning blade by a contact area of the cleaning blade with the image carrier. Even when the same load is applied to the cleaning blade, a contact area of the cleaning blade with the image carrier changes according to hardness, thickness, free length, and a shape of the cleaning blade. The “surface pressure” fluctuates according to the contact area, a material and a shape of the cleaning blade, a method of supporting the cleaning blade, and the like.
FIG. 19A depicts a cleaning blade 1a and FIG. 19B depicts a cleaning blade 1b with different shapes. The cleaning blade 1a has a planar shape, and is supported by a support member 2. A base end of the cleaning blade 1a adheres onto one side of the support member 2. A tip ridge portion 3a of the cleaning blade 1a is pressed against an image carrier 4. The cleaning blade 1b has a projected portion 5, and is supported by the support member 2. A base end of the cleaning blade 1b adheres onto one side of the support member 2. A tip ridge portion 3b of the cleaning blade 1b is pressed against the image carrier 4. In this case, the projected portion 5 abuts against the support member 2 to prevent the cleaning blade 1b from bending and the blade tip from being pushed away.
When a load is applied to the cleaning blade 1a, the cleaning blade 1a is deformed as shown in FIG. 19A. At a tip position of the support member 2, buckling is caused by concentration of stress. As a result, as shown in FIG. 20A, a tip “a” of the cleaning blade 1a curls back and a trunk portion “b” near the tip of the cleaning blade 1a comes into contact with a peripheral surface of the image carrier 4. Consequently, as shown in FIG. 21A, the load is dispersed and pressure distribution becomes small.
On the other hand, even if a load is applied to the cleaning blade 1b, the cleaning blade 1b is not deformed significantly as shown in FIG. 19B. As shown in FIG. 20B, the cleaning blade 1b is pressed against the image carrier 4 at a curled tip portion of the cleaning blade 1b, and a trunk portion of the cleaning blade 1b does not contact the image carrier 4. Consequently, as shown in FIG. 21B, a large pressure distribution with stress concentrated on the tip of the cleaning blade 1b is obtained.
Accordingly, depending on the shape of the cleaning blade, a contact area of the cleaning blade with the image carrier changes, and a surface pressure changes. This results in a difference in cleaning performance.
Japanese Patent Application Laid-Open No. 2000-75527 and Japanese Patent Application Laid-Open No. H11-237819 disclose cleaning devices that take a surface pressure into account. Japanese Patent Application Laid-Open No. 2002-268487 and Japanese Patent Application Laid-Open No. H5-19671 disclose cleaning devices that have an obtuse-angled blade edge. Japanese Patent Application Laid-Open No. H6-332350 discloses a cleaning device having a round blade edge with a curvature of 5 micrometers (μm) to 15 μm. Japanese Patent No. 2962843 discloses a cleaning device having a tapered blade.
However, none of the patent documents described above refers to a cleaning configuration that cleans off a toner of small and spherical particles with a low linear pressure and a high surface pressure.